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Glossary - PQ
PAR lamp See pressed reflector lamp. parking lamp a lighting device placed on a vehicle to indicate its presence when parked. partial diffusion† See incomplete diffusion. partly cloudy sky a sky that has 30 to 70% cloud cover. passing beams† See lower (passing) beams. pendant luminaire† See suspended (pendant) luminaire. perceived light source color the color perceived to belong to a light source. perceived object color† the color perceived to belong to an object resulting from characteristics of the object, of the incident light, and of the surround, the viewing direction, and observer adaptation. See object color. percent flicker a relative measure of the cyclic variation in output of a light source (percent modulation). It is given by the expression where A is the maximum and B is the minimum output during a single cycle. See Chapter 6, Light Sources. perfect diffusion that in which flux is uniformly scattered in accord with Lambert's cosine law. perimeter lights aeronautical ground lights provided to indicate the perimeter of a landing pad for helicopters. period life the time interval until lamps are replaced or luminaires are cleaned. peripheral vision the seeing of objects displaced from the primary line of sight and outside the central visual field. peripheral visual field that portion of the visual field that falls outside the region corresponding to the foveal portion of the retina. phosphor mercury lamp† see mercury-fluorescent lamp. phosphorescence the emission of light as the result of the absorption of radiation, and continuing for a noticeable length of time after excitation (longer than approximately 10−8 s). phot, ph a unit of illuminance equal to one lumen per square centimeter. The use of this unit is deprecated. photobiology a branch of biology that deals with the effects of optical radiation on living systems. photochemical radiation energy in the ultraviolet, visible, and infrared regions capable of producing chemical changes in materials. Note Examples of photochemical processes are accelerated fading tests, photography, photoreproduction, and chemical manufacturing. In many such applications a specific spectral region is of importance. photoelectric receiver† a device that reacts electrically in a measurable manner in response to incident radiant energy. photoflash lamp a lamp in which combustible metal or other solid material is burned in an oxidizing atmosphere to produce light of high intensity and short duration for photographic purposes. photoflood lamp an incandescent filament lamp of high color temperature for lighting objects for photography or videography. photometer an instrument for measuring photometric quantities such as luminance, luminous intensity, luminous flux, or illuminance. See densitomer, goniophotometer, illuminance (lux or footcandle) meter, integrating photometer, reflectometer, spectrophotometer, and transmissometer. photometry the measurement of quantities associated with light. Note Photometry can be either visual, in which the eye is used to make a comparison, or physical, in which measurements are made by means of physical receptors. photometric brightness† a term formerly used for luminance. photoperiod the environmental light/dark cycle to which living organisms may be exposed; for example, the natural cycle at the earth's equator of light (L) for 12 hours and darkness (D) for 12 hours. This is expressed as LD 12:12. photopic vision vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m2. See scotopic vision. photosynthetic irradiance irradiance within the wavelength band 400 to 700 nm. Unit: watts per square meter. photosynthetic photon flux density (PPFD) the number of photons per unit time and per unit area in the wavelength band 400 to 700 nm. Unit: micromoles per second and per square meter. Note (1) There are 6.0222 × 1023 photons in one mole; (2) this unit was formerly known as microeinsteins per second and per square meter. photosynthetically active radiation (PAR) photon flux in the wavelength band 400 to 700 nm. phototherapy the treatment of disease involving the use of optical radiation. physical photometer an instrument containing a physical receptor and associated filters that is calibrated so as to read photometric quantities directly. See visual photometer. pilot house control a mechanical means for controlling the elevation and train of a searchlight from a position on the other side of the bulkhead or deck on which it is mounted. Planck radiation law an expression representing the spectral radiance of a blackbody as a function of the wavelength and temperature. This law commonly is expressed by the formula where Lλ = the spectral radiance dIλ = the spectral radiant intensity dA' = the projected area (dA cos 8) of the aperture of the blackbody e = the base of natural logarithms (2.71828) T = absolute temperature c1L and c2 = constants designated as the first and second radiation constants. Note The symbol c1L is used to indicate that the equation in the form given here refers to the radiance L, or to the intensity I per unit projected area A’, of the source. Numeric values are commonly given not for c1L but for c1, which applies to the total flux radiated from a blackbody aperture, that is, in a hemisphere (2n sr), so that, with the Lambert cosine law taken into account, c1 = n c1L. The currently recommended value of c1 is 3.741832 × 10−16 W × m2, or 3.741832 × 10−12 W × cm2. Then c1L is 1.191062 × 10−16 W× m2 × sr−1, or 1.191062 × 10−12 W × cm2 × sr−1. If, as is more convenient, wavelengths are expressed in micrometers and area in square centimeters, then c1L = 1.191062 × 104 W × µm4 × cm−2 × sr−1, Lλ being given in W × cm−2 × sr−1 × µm−1. The currently recommended value of c2 is 1.438786 × 10−2 m × K. The Planck law in the following form gives the energy radiated from the blackbody in a given wavelength interval (λ1λ2): If A is the area of the radiation aperture or surface in square centimeters, t is the time in seconds, λ is the wavelength in micrometers, and c1 = 3.741832 × 104 W × µm4 × cm−2, then Q is the total energy in watt- seconds (joules), emitted from this area (that is, in the solid angle 2n) in time t within the wavelength interval (λ1λ2). planckian locus† See blackbody (planckian) locus. plano-convex spotlight a spotlight embodying a plano-convex lens and a lamp movable within the housing in relation to the lens in order to vary beam and field angles. point of fixation a point or object in the visual field at which the eyes look and upon which they are focused. point of observation for most purposes it can be assumed that the distribution of luminance in the field of view can be described as if there were a single point of observation located at the midpoint of the baseline connecting the centers of the entrance pupils of the two eyes. For many problems it is necessary, however, to regard the centers of the entrance pupils as separate points of observation for the two eyes. point-by-point method† a method of lighting calculation, now called the point method. point method a lighting design procedure for predetermining the illuminance at various locations in lighting installations by use of luminaire photometric data. The direct component of illuminance due to the luminaires and the interreflected component of illuminance due to the room surfaces are calculated separately. The sum is the total illuminance at a point. point source a source of radiation whose dimensions are sufficiently small, compared with the distance between the source and the irradiated surface, that these dimensions can be neglected in calculations and measurements. point vision the mode of vision of a small source of light such that the sensation is determined by its intensity rather than by its size. Point vision occurs with sources so small that their form or shape is not perceived and that they appear as points of light; this generally means less than 1-minute angular subtense. polarization† the process by which unpolarized radiation is polarized. It can be accomplished by either a reflection process or a transmission process. polarized radiation† radiation whose electromagnetic field, which is transverse, is oriented in defined directions. The polarization can be rectilinear, elliptic, or circular. pole (roadway lighting) a standard support generally used where overhead lighting distribution circuits are employed. portable lighting lighting involving equipment designed for manual portability. portable luminaire a lighting unit that is not permanently fixed in place. See table lamp ''and ''floor lamp. portable traffic control light a signaling light designed for manual portability that produces a controllable distinctive signal for purposes of directing aircraft operations in the vicinity of an aerodrome. position index, P a factor that represents the relative average luminance for a sensation at the borderline between comfort and discomfort (BCD) for a source located anywhere within the visual field. position lights aircraft aeronautical lights forming the basic, internationally recognized navigation light sys tem. Note The system is composed of a red light showing from dead ahead to 110° to the left, a green light showing from dead ahead to 110° to the right, and a white light showing to the rear through 140°. Position lights are also called navigation lights. prefocus lamp a lamp in which, during manufacture, the luminous element is accurately adjusted to a specified position with respect to the physical location element (usually the base). preheat (switch start) fluorescent lamp a fluorescent lamp designed for operation in a circuit requiring a manual or automatic starting switch to preheat the electrodes in order to start the arc. pressed reflector lamp an incandescent filament or electric-discharge lamp in which the outer bulb is formed of two pressed parts that are fused or sealed together; namely, a reflectorized bowl and a cover, which can be clear or patterned for optical control. Note Often called a projector or PAR lamp. primary (light) any one of three lights in terms of which a color is specified by giving the amount of each required to match it by additive combination. primary line of sight the line connecting the point of observation and the point of fixation. For a single eye, it is the line containing the point of fixation and the center of the entrance pupil. primary standards of light a light source by which the unit of light is established and from which the values of other standards are derived. This order of standard also is designated as the national standard. See national standard of light. Note A satisfactory primary (national) standard must be reproducible from specifications (see candela). Primary (national) standards usually are found in national physical laboratories such as the National Institute of Standards and Technology (NIST) in the United States. projection lamp a lamp with physical and luminous characteristics suited for projection systems (e.g., motion picture projectors, slide projectors, and microfilm viewers). projector a lighting unit that, by means of mirrors and lenses, concentrates the light to a limited solid angle so as to obtain a high value of luminous intensity. See floodlight, searchlight, ''and ''signaling light. protective lighting a system intended to facilitate the nighttime policing of industrial and other properties. proximal stimuli the distribution of illuminance on the retina constitutes the proximal stimulus. public way† any road, alley, or other similar parcel of land essentially open to the outside air, permanently appropriated for public use, and having a clear height and width of not less than 3 m (10 ft). pupil (pupillary aperture) the opening of the iris that admits light into the eye. See artificial pupil. Purkinje phenomenon the reduction in subjective brightness of a red light relative to that of a blue light when the luminances are reduced in the same proportion without changing the respective spectral distributions. In passing from photopic to scotopic vision, the curve of spectral luminous efficiency changes, the wavelength of maximum efficiency being displaced toward the shorter wavelengths. purple boundary the straight line drawn between the ends of the spectrum locus on a chromaticity diagram. quality of lighting pertains to the distribution of luminance in a visual environment. The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety, and aesthetics for the specific visual tasks involved. quantity of light (luminous energy), Q = jφdt the product of the luminous flux by the time it is maintained. It is the time integral of luminous flux. quartz-iodine lamp† an obsolete term for the tungsten halogen lamp. quick-flashing light a single flashing light at a frequency equal to or greater than 1 Hz. There is no agreed verbal differentiation between lights that flash at 1 Hz and those that flash more rapidly (a quick- flashing light can be a sequence of single flashes or a sequence of multiflick flashes, at 1-s intervals; there is no restriction on the ratio of the durations of the light to the dark periods).